Heat-resistant resins such as polyimide resins exhibit superior heat resistance and mechanical properties, and are consequently already in widespread use in the electronics field as surface protection films and interlayer insulating films for semiconductor elements. In recent years, screen printing methods and dispensing methods that do not require complex steps such as exposure, developing, and etching and the like are attracting considerable attention as image formation methods for polyimide-based resin films used as surface protection films, interlayer insulating films and stress relaxation materials.
Screen printing methods and dispensing methods typically use a heat-resistant resin paste with thixotropic properties that contains a base resin, a filler, and a solvent as components. Almost all of the heat-resistant resin pastes that have been developed to date have used a silica filler or an insoluble polyimide filler as a thixotropy-imparting filler, and as a result, large numbers of residual voids or air bubbles are left at the filler interface upon heating and drying, causing problems that include a deterioration in the film strength and inferior electrical insulation properties.
In order to overcome these problems, heat-resistant resin pastes that are capable of forming polyimide patterns with excellent properties have been developed by employing a special combination of an organic filler (soluble filler), a base resin, and a solvent in which the filler first dissolves upon heating and drying, and subsequently forms a co-soluble phase with the base resin to form a film (see Japanese Patent Publication No. 2,697,215 and Japanese Patent Publication No. 3,087,290).